/Engine/Source/Runtime/ApplicationCore/Public/GenericPlatform/GenericPlatformInputDeviceMapper.h
Get all currently active platform ids, anyone who has a mapped input device
The number of active platform users
virtual int32 GetAllActiveUsers ( [TArray](API\Runtime\Core\Containers\TArray)< [FPlatformUserId](API\Runtime\Core\Misc\FPlatformUserId) > & OutUsers ) const